1. Field of the Invention
The present invention relates to a nitride semiconductor structure and a semiconductor light emitting device including the same, especially to a nitride semiconductor structure that has a multiple quantum well structure formed by quaternary AlGaInN barrier layers and ternary InGaN well layers for reducing stress coming from lattice mismatch. The thickness of the well layer is ranging from 3.5 nm to 7 nm. At the same time, a better carrier confinement is provided and the internal quantum efficiency is improved. Thus the semiconductor light emitting device has a better light emitting efficiency.
2. Description of Related Art
Generally, a nitride light emitting diode is produced by forming a buffer layer on a substrate first. Then a n-type semiconductor layer, a light emitting layer and a p-type semiconductor layer are formed on the buffer layer in turn by epitaxial growth. Next use photolithography and etching processes to remove a part of the p-type semiconductor layer and a part of the light emitting layer until a part of the n-type semiconductor layer is exposed. Later a n-type electrode and a p-type electrode are respectively formed on the exposed n-type semiconductor layer and the p-type semiconductor layer. Thus, a light emitting diode device is produced. The light emitting layer has a multiple quantum well (MQW) structure formed by a plurality of well layers and barrier layers disposed alternately. The band gap of the well layer is lower than that of the barrier layer so that electrons and holes are confined by each well layer of the MQW structure. Thus electrons and holes are respectively injected from the n-type semiconductor layer and the p-type semiconductor layer to be combined with each other in the well layers and photons are emitted.
In the MQW structure, there are about 1-30 layers of well layers or barrier layers. The barrier layer is usually made of GaN while the well layer is made of InGaN. However, there is about 10˜15% lattice mismatch between GaN and InGaN that causes a large stress in the lattice. Thus a piezoelectric field is induced in the MQW structure by the stress. Moreover, during growth of InGaN, the higher indium composition, the larger the piezoelectric field generated. The piezoelectric field has a greater impact on the crystal structure. The stress accumulated is getting larger along with the increasing thickness during growth of InGaN. After the crystal structure being grown over a critical thickness, larger defects (such as V-pits) are present due to the stress, so that the thickness of the well layer has a certain limit, generally about 3 nm.
Moreover, in the MQW structure, band gap is tilted or twisted due to effects of a strong polarization field. Thus electrons and holes are separated and confined on opposite sides of the well layer, which leads to decrease the overlapping of the wave function of the electron hole pairs and further to reduce both radiative recombination rate and internal quantum efficiency of electron hole pairs.